The present invention relates to leuco dyes and a recording material employing the same, which is capable of yielding colored images having a sufficient absorption intensity in a near infrared region, especially used as a thermosensitive recording sheet and a pressure-sensitive recording sheet.
Recording materials using leuco dyes, as shown in Japanese Patent Publication No. 45-14039, are conventionally known and used in practice, for example, as pressure-sensitive recording sheets and thermosensitive recording sheets. Recently the above-mentioned pressure-sensitive recording sheets and thermosensitive recording sheets have been increasingly used.
There are generally used triphenylmethane-type leuco dyes, fluoran-type leuco dyes, phenothiazine-type leuco dyes and auramine-type leuco dyes. These leuco dyes are colored in a variety of different colors and are selectively used depending on the application.
However, such dyes have been developed, with an emphasis on the improvement of the color tone, that is, on the improvement of the absorption in the visible spectrum. Until recently, no dyes which can absorb the near infrared rays having a wavelength of 700 to 1,000 nm have been developed.
As a semiconductor laser becomes prevalent, a tendency to read recorded images such as bar codes by use of the semiconductor laser is growing, and a demand for a thermosensitive recording sheet and a pressure-sensitive recording sheet which can absorb light in a near infrared region is also increasing.
A variety of such leuco dyes capable of absorbing light in a near infrared region and a variety of thermosensitive recording sheets and pressure-sensitive recording sheets using the above leuco dyes have been proposed recently. For example, phthalide compounds containing one or two vinyl groups are shown in Japanese Laid-Open Patent Applications 51-121035, 57-167979 and 58-157779, fluorene compounds shown in Japanese Laid-Open Patent Applications 59-199757 and 60-226871, fluoran compounds shown in Japanese Laid-Open Patent Application 62-74687, and sulfonylmethane compounds shown in Japanese Laid-Open Patent Application 60-231766.
However, the above leuco dyes have the shortcoming that their absorption intensity in the near infrared region is not enough. In addition to this shortcoming, they have the shortcomings that the image formation stability is poor, which may readily cause discoloration of colored images, and accordingly such colored images cannot be easily read by optical readers.
Furthermore, Japanese Laid-Open Patent Application 62-173287 discloses a thermosensitive recording material in which a leuco dye of the following formula is employed: ##STR7## wherein R.sup.1 to R.sup.4 each represent a substituted or unsubstituted alkyl group; and R.sup.5 to R.sup.7 each represent a substituted or unsubstituted phenyl group.
The above leuco dye is similar in chemical structure to leuco dyes according to the present invention. However, the above recording material has the shortcomings that it is not resistant to light and the background of image areas is gradually discolored.